High-strength and high-ductility steel sheet and method of manufacturing the same

ABSTRACT

A high-strength and high-ductility steel sheet having a composition including, by weight, 1.0 to 1.4% C, 5.0 to 9.0% Mn, 2.0 to 8.0% Cr and the balance Fe, and unavoidable impurities. The steel sheet has an austenite structure formed at room temperature, and stacking fault energy is effectively controlled by the addition of Cr and N 2 . Mechanical twins are formed during the plastic deformation of the steel, thereby leading to high levels of work hardening, tensile strength and workability.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority from Korean Patent ApplicationNumber 10-2013-00125214 filed on Oct. 21, 2013, the entire contents ofwhich are incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength and high-ductilitysteel sheet, and more particularly, to an automotive steel sheet forwhich high workability is required, a high manganese (Mn) steel sheetapplicable as a shock absorber such as a vehicle bumper stiffener, and amethod of manufacturing the same.

2. Description of Related Art

Steel sheets applicable for vehicle bodies typically require highworkability. In order to satisfy this requirement, ultra-low carbonsteel has been mainly used for automotive steel sheets in the relatedart regardless of its low tensile strength ranging from 200 to 300 MPa,since it has high workability. Recently, a variety of methods forimproving vehicle fuel efficiency have been proposed in response toenvironmental issues such as air pollution. In particular, as a lighterweight of a vehicle is considered important to improve fuel efficiency,automotive steel sheets are required to have not only high workabilitybut also high strength.

In addition, the necessity of commercializing high-strength steel issignificantly increasing since some vehicle parts, such as bumperstiffeners or shock absorbers within the vehicle doors, are directlyrelated to the safety of the occupants. For example, the use ofultra-high-strength steel having a high tensile strength of typically780 MPa or higher and a large elongation is required.

Examples of such high-strength automotive steel include dual phase (DP)steel, transformation induced plasticity (TRIP) steel and twin inducedplasticity (TWIP) steel.

First, DP steel is produced by lowering the cooling end temperaturebelow the martensite start temperature (Ms) in the process of quenchinghot-rolled steel to room temperature so that part of austenitetransforms to martensite. Consequently, the dual phases of martensitetransformed from austenite and ferrite exist at room temperature. SuchDP steel can have a variety of mechanical properties due to anadjustment in the proportions of martensite and ferrite.

TRIP steel has improved workability, and is produced by partiallyforming retained austenite in a structure, followed by the martensitictransformation of austenite during the machining of components. AlthoughTRIP steel has the advantage of high strength caused by significanthardening through the martensitic transformation, its elongation is tooshort, which is problematic.

The hardening mechanism of either DP steel or TRIP steel is typicallybased on martensite, the hard phase. Martensite exhibits a significantincrease in hardening during plastic deformation, thereby enabling themanufacture of high-strength hot-rolled steel. However, the extremelylow ductility of martensite makes it difficult to achieve an elongationof 30% or more, which is problematic.

On the other hand, TWIP steel contains a large amount of Mn and has asingle austenite phase, which is stable at room temperature and allowsmechanical twins to form in the austenite structure during the machiningof component, thereby increasing the level of work hardening. That is,TWIP steel has an austenite structure instead of a ferrite structure asa matrix and has improved elongation through additional work hardeningby continuously generating mechanical twins in austenite grains toobstruct movement of the dislocations during plastic deformation.Further, TWIP steel may have large elongation and high tensile strengthdue to the mechanical twins causing a high level of work hardening. Inparticular, TWIP steel has elongation 50% larger than that ofconventional DP steel or TRIP steel and is thus preferably applied tosteel sheets for automobiles.

However, current TWIP steel has a high Mn content ranging from about 18%to about 30% in order to guarantee austenite stability and adjuststacking fault energy, and requires the addition of large amounts ofaluminum or silicon together with manganese, causing a significantincrease in material and manufacturing costs. Moreover, there is a needfor the development of TWIP steel having a low Mn content in order toavoid an additional increase in manufacturing costs caused byvolatilization of Mn or temperature decrease during a steelmanufacturing process or continuous casting process. Furthermore, interms of mechanical properties, since currently developed TWIP steel hasa low yield strength of about 300 MPa and a tensile strength of 1 GPa orless, there is a need for steel sheets which have a higher strengthwithout deteriorating elongation.

The information disclosed in the Background of the Invention section isprovided only for better understanding of the background of theinvention and should not be taken as an acknowledgment or any form ofsuggestion that this information forms a prior art that would already beknown to a person skilled in the art.

BRIEF SUMMARY OF THE INVENTION

Various aspects of the present invention provide a steel sheet able toovercome the problems of dual phase (DP) steel, transformation inducedplasticity (TRIP) steel and twin induced plasticity (TWIP) steel of therelated art.

Also provided is a steel sheet able to achieve both high strength andhigh ductility while reducing the content of Mn.

In an aspect of the present invention, provided is a high-strength andhigh-ductility steel sheet having a composition including, by weight,0.8 to 1.4% C, 5.0 to 10.0% Mn, 2.0 to 8.0% Cr and the balance Fe, andunavoidable impurities.

The steel sheet may be formed of twinning-induced plasticity steel.

The steel sheet may exclude chromium carbides.

The steel sheet may be a hot-rolled steel sheet heat-treated at atemperature of 1100° C. or higher.

The hot-rolled steel may be manufactured by heating the steel sheet at atemperature of 1100° C. or higher, hot-rolling the heated steel sheet ata temperature of 900° C. or higher, and subsequently cooling thehot-rolled steel sheet by air cooling or forced cooling.

The steel sheet may be a cold-rolled annealed steel manufactured bycold-rolling the hot-rolled steel.

The cold-rolled annealed steel may be manufactured by cold-rolling thecooled steel sheet with a thickness reduction ratio of 30% or greater atroom temperature, annealing the cold-rolled steel sheet at a temperatureof 800° C. or higher, and subsequently cooling the annealed steel sheetby air cooling or forced cooling.

In the steel sheet, a value obtained by multiplying the tensile strengthand the total elongation of the steel sheet may be 30,000 MPa % orgreater.

The amount of the Cr may range from 4.0 to 7.0% by weight.

The composition may further include, by weight, 0.1 to 2.0% Al.

In an aspect of the present invention, provided is a method ofmanufacturing a high-strength and high-ductility steel sheet. The methodincludes the following operations of: preparing a steel sheet having acomposition comprising, by weight, 0.8 to 1.4% carbon, 5.0 to 10.0%manganese, 2.0 to 8.0% chromium and the balance iron, and unavoidableimpurities; heating the steel sheet at a temperature of 1100° C. orhigher; hot-rolling the heated steel sheet at a temperature of 900° C.or higher; and cooling the hot-rolled steel sheet by air cooling orforced cooling.

The method may further include the operations of: cold-rolling thecooled steel sheet with a thickness reduction ratio of 30% or greater atroom temperature; annealing the cold-rolled steel sheet at a temperatureof 800° C. or higher; and cooling the annealed steel sheet by aircooling or forced cooling.

The reasons why the composition of the steel sheet according to theinvention is limited as above will be described as follows.

Mn: 5.0 to 10.0 wt %

Twin induced plasticity (TWIP) steel must have an austenite phase atroom temperature after being hot rolled, since mechanical twins areformed in an austenite matrix at room temperature during plasticdeformation. Mn is an austenite stabilizing alloying element that allowsaustenite, i.e. a high-temperature phase in the Fe—C binary phasediagram, to form at room temperature. At a Mn content less than 5% byweight, the austenite phase becomes remarkably unstable. After hotrolling, a ferrite or martensite phase is formed in the austenitestructure during cooling, whereby no mechanical twins can be formedduring plastic deformation. Accordingly, the Mn content is required tobe 5.0% by weight or greater.

At a Mn content exceeding 10.0% by weight, it is possible to produce asingle austenite phase and form mechanical twins at room temperature.However, there are no significant differences from related-art TWIPsteel. Thus, some problems, such as expensive manufacturing costs,degraded weldability and inserts, may still occur. Therefore, accordingto the invention, the Mn content is limited to the range from 5.0 to9.0% by weight.

C: 0.8 to 1.4 wt %

At a Mn content less than 5.0% by weight, Fe—Mn binary alloys have εmartensite or α′ martensite partially formed instead of a singleaustenite phase at room temperature. In order to form a single austenitephase structure at room by overcoming this problem, C can be desirablyadded as an inexpensive and highly effective austenite stabilizingelement. At a C content less than 0.8% by weight, it is difficult toobtain a single austenite phase during cooling after hot rolling sinceaustenite stability is still insufficient. Even if the single austenitephase is obtained at room temperature, phase transformation occurs fromaustenite to martensite during plastic deformation to form TRIP steel,due to insufficient austenite stability. Consequently, TWIP steelintended in the present invention cannot be obtained. On the other hand,at a C content exceeding 1.4% by weight, stable austenite can beobtained at room temperature, but cementite precipitation occurs,thereby decreasing elongation and reducing weldability. Even if thecooling rate is controlled after annealing heat treatment, it is stilldifficult to control the precipitation of carbides. Since C increasesstacking fault energy, a large C content makes it difficult to formmechanical twins during deformation. Accordingly, it is preferable thatthe C content is limited to the range from 1.0 to 1.4% by weight.

Cr: 2.0 to 8.0 wt %

Cr has been mainly used in stainless steel since it improves corrosionresistance. Cr not only functions as a ferrite stabilizing element, butalso stabilizes austenite by lowering martensite transformationtemperature when added to austenite steel. When Cr is added to theFe—Mn—C ternary system, Cr can control martensite transformation topromote the formation of mechanical twins in the austenite matrix. Incontrast, at a Cr content less than 2.0% by weight, austenite stabilityis insufficient, and strain induced martensite is formed instead ofmechanical twins during plastic deformation, thereby producing TRIPsteel.

Although it is known that Cr decreases stacking fault energy instainless steel like Mn. In contrast, in Fe—Mn—C ternary alloys, Crincreases stacking fault energy. If the Cr content exceeds 8.0% byweight, the stacking fault energy of austenite becomes excessively high.Hardening is caused by simple perfect dislocation movement instead ofmechanical twins during plastic deformation, making it difficult toachieve either high strength or high ductility. In addition, since Cr isa ferrite stabilizing element, the Cr content exceeding 8.0% by weightmay cause partial formation of ferrite during hot rolling. Furthermore,the use of a large amount Cr significantly increases manufacturingcosts. Therefore, it is preferable that the Cr content is limited to therange from 2.0 to 8.0% by weight.

The composition of the steel sheet according to the present inventionmay selectively include Al. Hydrogen permeation into the steel sheetaccording to the invention may cause problems involving hydrogenembrittlement. These problems can be effectively overcome by theaddition of Al. Although the Al content is not specifically limited, Alis typically added at an amount of 2.0% or less by weight.

As set forth above, the steel sheet according to the present inventionhas the austenite structure formed at room temperature while containinga small amount of Mn. In addition, the stacking fault energy iseffectively controlled. Therefore, mechanical twins are formed duringthe plastic deformation of steel, leading to high levels of workhardening, tensile strength and workability. That is, in the steel sheetaccording to the present invention, the product of the tensile strengthand the total elongation (TS×El) has a very large value of 30,000MPa %or greater. The product of the tensile strength and the total elongationis substantially the same and manufacturing costs are significantlyreduced comparing to those of related-art TWIP steel having a Mn contentof about 20% by weight.

In addition, the high manganese nitrogen-containing steel sheet can beimplemented as a variety of steel sheets, such as a hot-rolled steelsheet and a cold-rolled annealed steel sheet.

The methods and apparatuses of the present invention have other featuresand advantages that will be apparent from, or are set forth in greaterdetail in the accompanying drawings, which are incorporated herein, andin the following Detailed Description of the Invention, which togetherserve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscopy (SEM) picture of an annealedsteel sheet according to a comparative example of the present invention;

FIG. 2 is an SEM picture of an annealed steel sheet according to anexample of the present invention; and

FIG. 3 is an SEM picture illustrating mechanical twins formed in a steelsheet according to an example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to exemplary embodiments of a steelsheet and a method of manufacturing the same according to the presentinvention.

First, a steel sheet according to an exemplary embodiment of theinvention has a composition including, by weight, 0.8 to 1.4% C, 5.0 to10.0% Mn, 2.0 to 8.0% Cr and the balance Fe, and unavoidable impurities.

The present invention has been devised in order to overcome the problemsof twin induced plasticity (TWIP) steel of the related art. In the steelsheet according to this embodiment, suitably adjusted amounts of C, Crand the like are added, whereas the Mn content is lowered to be, byweight, 10% or less. This can consequently obtain a single austenitephase at room temperature while minimizing the Mn content. Accordingly,the composition of the steel sheet according to the present inventionincludes these alloy elements, thereby achieving a large elongation andhigh yield strength and tensile strength than related-art TWIP steelwhile reducing the contents of expensive alloy elements, such as Mn andAl, below those of the related-art TWIP steel.

Specifically, the steel sheet according to an exemplary embodiment ofthe invention includes, by weight, 5.0 to 10.0% Mn. Since TWIP steel hasmechanical twins formed in an austenite matrix at room temperatureduring plastic deformation, it is important to expand an austeniteregion from high temperature to room temperature on the Fe—C phasediagram. In addition, stacking fault energy must be about 50 mmJ/m² orless in order to form mechanical twins during plastic deformation.According to the present invention, Mn is employed as an austenitestabilizing element.

At a Mn content less than 5% by weight, austenite stability issignificantly reduced, allowing ferrite or martensite to form in theaustenite region during cooling after hot rolling. In addition, at theMn content less than 5% by weight, the stacking fault energy of theaustenite phase excessively increases, thereby making it difficult toform mechanical twins.

On the other hand, if the Mn content is too great, the stacking faultenergy excessively increases, such that plastic deformation may occur inthe austenite phase. In addition, since one object of the invention isto minimize the content of expensive Mn, it is preferable to maintainthe Mn content within 10% by weight according to this embodiment inorder to reduce manufacturing costs as low as possible.

In addition, the composition of the steel sheet according to thisembodiment includes, by weight, 0.8 to 1.4% C. Here, Fe—Mn binary alloyshave ε or α′ martensite partially formed therein instead of a singleaustenite phase at room temperature. Therefore, according to thisembodiment, C is added as an inexpensive and effective austenitestabilizing element in order to obtain a single austenite phasestructure at room temperature.

At a C content less than 0.8% by weight, it is difficult to obtain asingle austenite phase during cooling after hot rolling since austenitestability is still insufficient. Even if the single austenite phase isobtained at room temperature, phase transformation occurs from austeniteto martensite during plastic deformation to form TRIP steel, due toinsufficient austenite stability. Consequently, TWIP steel intended inthe present invention cannot be obtained.

On the other hand, at a C content exceeding 1.4% by weight, stableaustenite can be obtained at room temperature, but carbide precipitationoccurs, thereby decreasing elongation and reducing weldability. Even ifthe cooling rate is controlled after annealing heat treatment, it isstill difficult to control the precipitation of carbides. Since C is anelement that increases stacking fault energy, a large C content makes itdifficult to form mechanical twins during deformation, which isproblematic. Accordingly, it is preferable that the C content is limitedto the range from 1.0 to 1.4% by weight.

Furthermore, the composition of the steel sheet according to thisembodiment includes, by weight, 2.0 to 8.0% Cr. Cr has been mainly usedin stainless steel since it improves corrosion resistance. Cr not onlyfunctions as a ferrite stabilizing element, but also stabilizesaustenite by lowering martensite transformation temperature when addedto austenite steel. When Cr is added to the Fe—Mn—C ternary system, Crcan control martensite transformation to promote the formation ofmechanical twins in the austenite matrix. In contrast, at a Cr contentless than 2.0% by weight, austenite stability is insufficient, andstrain induced martensite is formed instead of mechanical twins duringplastic deformation, thereby producing TRIP steel.

Although it is known that Cr decreases stacking fault energy instainless steel like Mn. In contrast, in Fe—Mn—C ternary alloys, Crincreases stacking fault energy. If the Cr content exceeds 8.0% byweight, the stacking fault energy of austenite becomes excessively high.Hardening is caused by simple perfect dislocation movement instead ofmechanical twins during plastic deformation, making it difficult toachieve either high strength or high ductility. In addition, since Cr isa ferrite stabilizing element, the Cr content exceeding 8.0% by weightmay cause partial formation of ferrite during hot rolling. Furthermore,the use of a large amount Cr significantly increases manufacturingcosts. Therefore, it is preferable that the Cr content is limited to therange from 2.0 to 8.0% by weight.

EXAMPLES

3 mm thick steel sheets, the chemical compositions of which arepresented in Table 1 below, were formed by heating at a temperature of1100° C. or higher, followed by hot rolling at a temperature of 900° C.or higher. The steel sheets were subsequently subjected to oil coolingor water cooling, thereby manufacturing steel samples (InventiveExamples 1 to 3 and Comparative Examples 1 to 7). In addition, part ofthe hot-rolled steel samples were subjected to annealing heat treatmentat 800 to 1200° C. for 5 to 10 minutes, followed by oil cooling or watercooling.

TABLE 1 Composition (wt %) Sample No. C Mn Cr Remarks Inventive Ex. 11.22 7.34 3.03 Water cooled after annealed at 1200° C. Inventive Ex. 21.18 7.23 4.89 Water cooled after annealed at 1200° C. Inventive Ex. 31.23 7.42 6.92 Water cooled after annealed at 1200° C. Comp. Ex. 1 1.227.34 3.03 Water cooled after annealed at 1000° C. Comp. Ex. 2 1.18 7.244.89 Water cooled after annealed at 1000° C. Comp. Ex. 3 1.23 7.42 6.92Water cooled after annealed at 1000° C. Comp. Ex. 4 1.22 7.34 3.03 Watercooled after annealed at 800° C. Comp. Ex. 5 1.18 7.24 4.89 Water cooledafter annealed at 800° C. Comp. Ex. 6 1.23 7.42 6.92 Water cooled afterannealed at 800° C. Comp. Ex. 7 1.19 8.08 <1 wt % Water cooled afterannealed at 1000° C.

TABLE 2 YS¹ TS² El³ TS × El Sample No. (MPa) (MPa) (%) (MPa %) RemarksInventive 410 829 38.6 31999 Water cooled after Ex. 1 annealed at 1200°C. Inventive 484 953 39.7 37834 Water cooled after Ex. 2 annealed at1200° C. Inventive 515 1014 38.0 38532 Water cooled after Ex. 3 annealedat 1200° C. Comp. Ex. 1 512 1074 22.2 23843 Water cooled after annealedat 1000° C. Comp. Ex. 2 595 1211 22.0 26642 Water cooled after annealedat 1000° C. Comp. Ex. 3 614 1162 15.6 18127 Water cooled after annealedat 1000° C. Comp. Ex. 4 893 1054 2.1 2213 Water cooled after annealed at800° C. Comp. Ex. 5 984 1625 6.3 10237 Water cooled after annealed at800° C. Comp. Ex. 6 1021 1557 5.5 8563 Water cooled after annealed at800° C. Comp. Ex. 7 371 830 23.1 19173 Water cooled after annealed at1000° C. Note) YS¹: Yield Strength, TS²: Tensile Strength, El³: TotalElongation

The strength and elongation were measured from the samples manufacturedby the above-described process, and the results are presented in Table2. As presented in Table 2, tensile properties were significantlydifferent according to the annealing temperatures even at the samecomposition. This relates to chromium carbides formed by the addition ofCr. As illustrated in FIG. 1, the microstructure of comparative steel 2has a large amount of carbides formed within the grains and matrix. Incontrast, no chromium carbides were observed in the annealedmicrostructure of inventive steel 2 having the same composition.Therefore, when a low annealing temperature or a slow cooling ratecauses carbide precipitation within the austenite matrix, C and Cr inaustenite have a small solubility, thereby degrading the stability ofaustenite. Since carbides are already precipitated to coarse sizes,tensile properties are subjected to an adverse effect even in the sametype of steel. As a result, as presented in Table 2, the strength Xelongation of each of Inventive Examples is 30,000 MPa % or greater,whereas the strength X elongation of each of Comparative Examples 1, 2and 3 is smaller than 30,000 MPa %.

Comparative Examples 4, 5 and 6 have a greater amount of precipitationswithin the austenite matrix than the examples annealed at 1000° C.,since Comparative Examples 4, 5 and 6 were annealed at 800° C. where theprecipitation of chromium carbides is most active. Since the stabilitywithin the austenite matrix is significantly lowered, stress inducedmartensite is formed during plastic deformation. It is appreciated thatthe maximum tensile stress is very high but the elongation is veryshort.

Even in the case where Mn or C is within the range of the inventiveexamples, if Cr is added at an amount smaller than the referencecontent, no superior tensile properties are obtained, as apparent fromComparative Example 7. The microstructure of Inventive Example 3 duringplastic deformation was observed using a scanning electron microscope(SEM) in order to examine the formation of mechanical twins duringplastic deformation. As illustrated in FIG. 3, well-developed mechanicaltwins are appreciated.

As set forth above, the exemplary embodiments of the high-strength andhigh-ductility steel sheet and the method of manufacturing the sameaccording to the present invention have been described in detail.However, a person skilled in the art can make various alternatives andmodifications of the exemplary embodiments. It should be thereforeunderstood that the scope of the present invention shall be defined bythe Claims appended hereto and their equivalents.

What is claimed is:
 1. A high-strength and high-ductility steel sheethaving a composition comprising, by weight, 0.8 to 1.4% carbon, 5.0 to10.0% manganese, 2.0 to 8.0% chromium and the balance iron, andunavoidable impurities.
 2. The steel sheet according to claim 1,comprising twinning-induced plasticity steel.
 3. The steel sheetaccording to claim 1, excluding chromium carbides.
 4. The steel sheetaccording to claim 1, comprising a hot-rolled steel sheet heat-treatedat a temperature of 1100° C. or higher.
 5. The steel sheet according toclaim 4, wherein the hot-rolled steel is manufactured by heating thesteel sheet at a temperature of 1100° C. or higher, hot-rolling theheated steel sheet at a temperature of 900° C. or higher, andsubsequently cooling the hot-rolled steel sheet by air cooling or forcedcooling.
 6. The steel sheet according to claim 4, comprising acold-rolled annealed steel manufactured by cold-rolling the hot-rolledsteel.
 7. The steel sheet according to claim 6, wherein the cold-rolledannealed steel is manufactured by cold-rolling the cooled steel sheetwith a thickness reduction ratio of 30% or greater at room temperature,annealing the cold-rolled steel sheet at a temperature of 800° C. orhigher, and subsequently cooling the annealed steel sheet by air coolingor forced cooling.
 8. The steel sheet according to claim 1, wherein avalue obtained by multiplying a tensile strength with a total elongationis 30,000 MPa % or greater.
 9. The steel sheet according to claim 1,wherein an amount of the chromium ranges from 4.0 to 7.0% by weight. 10.The steel sheet according to claim 1, wherein the composition furthercomprises, by weight, 0.1 to 2.0% aluminum.
 11. A method ofmanufacturing a high-strength and high-ductility steel sheet, the methodcomprising: preparing a steel sheet having a composition comprising, byweight, 0.8 to 1.4% carbon, 5.0 to 10.0% manganese, 2.0 to 8.0% chromiumand the balance iron, and unavoidable impurities; heating the steelsheet at a temperature of 1100° C. or higher; hot-rolling the heatedsteel sheet at a temperature of 900° C. or higher; and cooling thehot-rolled steel sheet by air cooling or forced cooling.
 12. The methodaccording to claim 11, further comprising: cold-rolling the cooled steelsheet with a thickness reduction ratio of 30% or greater at roomtemperature; annealing the cold-rolled steel sheet at a temperature of800° C. or higher; and cooling the annealed steel sheet by air coolingor forced cooling.
 13. The method according to claim 11, wherein anamount of the chromium ranges, by weight, 4.0 to 7.0%.
 14. The methodaccording to claim 11, wherein the composition further comprises, byweight, 0.1 to 2.0% aluminum.